It has become common to monitor machinery and apparatus of various sorts remotely from the location of such apparatus utilizing automated sensing and measuring equipment, computerized analysis, and automated transmissions of alarms and data via modem to central locations where response can be initiated. An example is in a city in which a plurality of buildings having multiple elevators each may transmit information concerning degradation or failure of an elevator to a single location in a city, where response may or may not be initiated, and additionally sending such information from a number of cities to a single central location for further response.
The use of modems on ordinary telephone lines is, however, subject to receipt of spurious, unauthorized messages, as a result of intentional mischief, and as a result of misdialed or misdirected phone calls. Some realistic consequences of spurious messages include corruption of data bases, including erasure of stored data, such as maintenance data, unwarranted access to control over building equipment, such as lights, heating and ventilating and the like.
An obvious, similar situation exists with respect to fax machines, which are more and more subject to auto dialing, advertising, and other unwanted input. In cases where a fax machine is desired to be utilized only in a priority fashion with more than just a few, predetermined senders (which could be controlled by polling numbers), a need exists to preclude responding to unwanted calls.